This invention relates to optical transmitters and in particular to laser diode slope compensation.
Slope variation of a laser diode is the variation in output optical power with drive current, above threshold, which occurs with variations in temperature and over time. For optical fibre transmission, a slope variation compensation circuit must be used in order to maintain a constant extinction ratio of the transmitted optical signal.
Slope variation compensation may be achieved by superimposing a low frequency ripple onto the optical carrier to "measure" the slope variation of optical output versus drive current (within the lasing mode). The ripple is detected by a back facet monitor photodiode, which may be configured within a feedback, feedforward, or a combination of feedback and feedforward, arrangement to compensate for slope variation. The disadvantage of this method is that it depends on the linearity of laser light against drive current slope at the high current part of the slope, where non-linearity are most likely to occur. Another approach is to peak detect the digital optical signal from the back facet monitor photodiode and compare the digital "1" and "0" with their respective reference voltages.